The invention relates generally to non-destructive evaluation of wind turbine blades, and more specifically, to stress measurements techniques.
Static and dynamic loading are typically employed in wind turbine blades for testing strength of the blades. While a static load is applied in the former case, a varying load or a cyclical load is applied in dynamic loading. Typically, various techniques are employed to measure stress on the blades during such a loading process.
A commonly known stress measurement technique employs strain gauges mounted on components such as blades, to measure stress indirectly. Generally, the blades are instrumented with strain gauges to measure strains at various locations. For example, about 120 strain gauges are mounted on a 37 m blade and about 20% of those are mounted on inside of the blade. Such measurements are compared to design specification limits and an alarm is set in an event that the measurements exceed specification limits
However, the bonding of the strain gauges to the blade via an epoxy measure strain on a surface of the blade at a fixed location. Such measurements are affected by factors such as, but not limited to, rigidity of adhesion and orientation of strain gauges. Additionally, the strain gauge measurements may be affected by temperature variations. Furthermore, presence of wrinkles, delaminations or other defects on the blades may cause a change in the actual strain.
Accordingly, an improved stress measurement technique is needed to address the one or more aforementioned issues.